Contact lenses made from silicone-containing materials have been investigated for a number of years. Such material can generally be subdivided into two major classes: hydrogels and non-hydrogels. Non-hydrogels do not absorb appreciable amounts of water, whereas hydrogels can absorb and retain water in an equilibrium state. Hydrogels generally have a water content greater than about five weight percent and more commonly between about ten to about eighty weight percent. Regardless of their water content, both non-hydrogel and hydrogel silicone contact lenses tend to have relatively hydrophobic, non-wettable surfaces.
Those skilled in the art have long recognized the need for rendering the surface of contact lenses hydrophilic or more hydrophilic. Increasing the hydrophilicity of the contact-lens surface improves the wettability of the contact lenses with tear fluid in the eye. This in turn improves the wear comfort of the contact lenses. In the case of continuous-wear lenses, the surface is especially important. The surface of a continuous-wear lens must be designed, not only for comfort, but to avoid adverse reactions such as corneal edema, inflammation, or lymphocyte infiltration.
Silicone lenses have been subjected to plasma surface-treatment to improve their surface properties, for example, in order to make the surface more hydrophilic, deposit-resistant, scratch-resistant, and the like. Examples of common plasma surface treatments include subjecting contact lens surfaces to a plasma comprising: (1) an inert gas or oxygen as, for example, in U.S. Pat. Nos. 4,055,378; 4,122942; and 4,214,014; (2) various hydrocarbon monomers as, for example, U.S. Pat. No. 4,143,949; and (3) combinations of oxidizing agents and hydrocarbons, for example, water and ethanol as in WO 95/04609 and U.S. Pat. No. 4,632,844. Sequential plasma surface treatments are also known, such as those comprising a first treatment with a plasma of an inert gas or oxygen, followed by a hydrocarbon plasma. For example, U.S. Pat. No. 4,312,575 to Peyman et al. discloses a process for providing a barrier coating on a silicone or polyurethane lens wherein the lens is subjected to an electrical glow discharge (plasma) involving a hydrocarbon atmosphere followed by oxygen in order to increase the hydrophilicity of the lens surface.
With oxidizing plasma, for example O.sub.2 (oxygen gas), water, hydrogen peroxide, air, or the like, the plasma tends to etch the surface of the lens, creating radicals and oxidized functional groups. When used as the sole surface treatment, such oxidation renders the surface of a silicone lens more hydrophilic. However, the coverage of such surface treatment may not be complete and the bulk properties of the silicone materials may remain apparent at the surface of the lens, (e.g., silicone molecular chains adjacent the lens surface are capable of rotating thus exposing hydrophobic groups to the outer surface). Such coatings have been found to be thin, whereas thicker coatings tend to crack. Hydrocarbon plasmas, on the other hand, deposit a thin carbon layer (e.g. from a few Angstroms to several thousand Angstroms thick) upon the surface of the lens, thereby creating a barrier between the underlying silicone materials and the outer lens surface. Following deposition of a thin carbon layer on the lens to create a barrier, plasma oxidation can be employed to increase the hydrophilicity of the surface.
Although known surface treatments can be effective in improving the surface properties of non-hydrogel silicone contact lenses, problems are encountered when such treatments are applied to hydrogel lens. Silicone hydrogel lenses are coated in an unhydrated state, but subsequently hydrated during manufacture and prior to use. This hydration causes the lens to dramatically swell, commonly from about ten to about twenty percent in volume, depending upon the water content of the lens. Such swelling of the lens commonly may cause plasma coatings to crack, delaminate, and/or rub off. Furthermore, plasma coatings can compromise lens hydration by not permitting proper lens expansion and thereby causing lens destruction.
Various patents disclose the grafting of hydrophilic or otherwise biocompatible polymers to the surface of a contact lens in order to render the lens more biocompatible. For example, U.S. Pat. No. 5,805,264 to Jannsen et al. teaches the graft polymerization of an ethylenically unsaturated oligomer or polymer onto the surface of a lens in the presence of a cross-linking agent, following the plasma treatment of the lens to form hydroperoxy groups on the surface of the lens. U.S. Pat. No. 5,260,093 to Kamel et al. discloses covalently grafting a polymeric biocompatible material to the surface of a substrate by radio frequency plasma induction. U.S. Pat. No. 5,206,298 to Kawaguchi discloses the graft polymerization of 2-hydroxyethyl methacrylate by using a polymeric polymerization initiator comprising a peroxyfumurate.
The graft copolymerization onto a silicone substrate material has been problematic and unsatisfactory for several reasons. One serious complication has been the simultaneous and undesired homopolymerization of the vinylic monomer being grafted, resulting in wasted polymer that must be removed and discarded. Another problem has been the depth and density control of the graft. Grafts of excessive depth, grafts of insufficient density to achieve the desired property modification, and the swelling and degradation of the medical-device substrate during the process has occurred. Graft polymerization into interior portions of the substrate beneath the surface of the substrate can cause distortion of the medical device.
In view of the above, it would be desirable to find an improved optically clear, hydrophilic coating for the surface of a silicone hydrogel contact lens or other medical device that does not suffer from the aforementioned disadvantages and which is economical to produce. It would be further desirable to obtain a coating for a contact lens or other medical device that is more comfortable or biocompatible for longer periods of time, which coating, in the case of a contact lens, is simultaneously tear-wettable and highly permeable to oxygen. It would be desirable if such a biocompatibilized lens was capable of continuous wear overnight, preferable for a week or more without adverse effects to the cornea.